From the 2.5 manual:
2.15.9.9 MSH-9 Message Type (MSG) 00009
Components: ^ ^
Definition: This field contains the message type, trigger event, and the message structure ID for the message.
Refer to HL7 Table 0076 – Message type for valid values for the message type code. This table contains values such as ACK, ADT, ORM, ORU etc.
Refer to HL7 Table 0003 – Event type for valid values for the trigger event. This table contains values like A01, O01, R01 etc.
Refer to HL7 Table 0354 – Message structure for valid values for the message structure. This table contains values such as ADT_A01, ORU_R01, SIU_S12, etc. .
The receiving system uses this field to recognize the data segments, and possibly, the application to which to route this message. For certain queries, which may have more than a single response event type, the second component may, in the response message, vary to indicate the response event type. See the discussion of the display query variants in chapter 5.